Many berries, including blueberries, strawberries, cranberries, raspberries, black currants and others, contain naturally occurring chemical constituents with antioxidant activity and may therefore be termed "antioxidant-rich berries". Antioxidants are chemicals that can inhibit oxidation reactions. Oxidation reactions are known to produce harmful chemicals within living animals, including humans. Chemically, oxidation is an event in which a compound loses electrons. In biological systems, unsaturated lipids are important constituents that are highly susceptible to oxidation reactions, especially autoxidation reactions, that is oxidation reactions with molecular oxygen.
Autoxidation reactions of complex compounds typically occur in a series or chain-reaction, which can be divided into three stages described as initiation, propagation and termination stages (Pokorny, J. 1999. Antioxidants in Food Preservation. p. 309-337, IN "Handbook of Food Preservation" M. S. Rahman, ed. Marcel Dekker Inc., N.Y.). Initiation requires a free radical, that is a compound with an unpaired electron. Free radicals may arise from a number of physiological or degradative reactions within biological tissues and materials. Free radicals are very reactive and quickly react with unsaturated lipids, oxygen and other compounds to form degradation products, some of which are themselves free radicals. This is called the propagation stage of autoxidation. Eventually the chain reaction may terminate when all oxidizable materials or free radicals are consumed.
Pro-oxidants are compounds that tend to initiate and promote the oxidation sequence and include metals such as iron and copper. Antioxidants are substances that can delay the onset or slow the rate of oxidation of autoxidizable materials (Nawar, W. 1985 Lipids, p. 225-320. IN "Food Chemistry" O. R. Fennema, ed. Marcel Dekker Inc. N.Y.). Antioxidants can function by a number of mechanisms such as chelating metals to inhibit their pro-oxidant activity or by combining with and "quenching" free radicals. Other compounds with antioxidant activity such as ascorbic acid, act as synergists with other antioxidants, often re-activating spent antioxidants by reducing them back into an active form. Free radicals are also generated as byproducts of many physiological reactions within living organisms and for this reason, a balance must be maintained in the human body between pro-oxidants and antioxidants.
Due to the toxicity of many autoxidation products, antioxidants are considered desirable in the human diet. Many health benefits have been associated with antioxidants in foods, including anti-mutagenicity, anti-carcinogenicity and anti-aging (Cook and Samman, 1996. Flavonoids chemistry, metabolism, cardioprotective effects and dietary sources. Nutritional Biochemistry 7:66-76; Huang and Frankel, 1997. Antioxidant activity of tea catechins in different lipid systems. J. Agricultural and Food Chemistry. 45: 3033-3038). Although synthetic chemical antioxidants are known, natural antioxidants in familiar foods are in great demand due to their long historical record in the human diet and presumed safety. Natural antioxidants which have be identified in common foods include phenolic compounds such as tocopherols and flavonids, including anthocyanins, as well as carotenoids, amino acids, and ascorbic acid (Pokomy, J. 1999. ).
Blueberries (a prime example of antioxidant-rich berries) are the fruit of plants belonging to the genus Vaccinium, including V. corymbosum, V. ashei and V. augustifolium and grow throughout North America. They have been used as food since prehistoric times and today are an important food crop. Commercially marketed blueberries include both wild, low-bush blueberries (e.g. V. augustifolium) which grow primarily in Maine, Nova Scotia and Quebec, and cultivated, high-bush blueberries (e.g. V. corymbosum and V. ashei), grown principally in British Columbia, Michigan and New Jersey.
Blueberries have a brief harvest season of about one month, after which fresh blueberries can only be stored refrigerated for a maximum of 6 weeks. Therefore further processing is desirable to extend shelf-life. Although large amounts of blueberries are frozen for preservation, frozen storage-life is only about 6 months, after which the fruit develop texture problems such as woodiness and grittiness (Sullivan et al., 1982. Dehydrated blueberries by the continuous explosion-puffing method. Journal of Food Science 47: 445-448). Dehydration is another popular preservation method for blueberries. If the water activity of the fruit is reduced by dehydration to below 0.60, spoilage micro-organisms are unable to grow and a storage life greater than 6 months can be achieved. Furthermore, dehydrated blueberries do not require energy-intensive refrigerated storage and are lighter and less fragile during transportation.
Most commercial dehydration of blueberries is accomplished by hot air forced-convection drying in which heated dry air is passed over or through a bed of the fruit. A small portion of the blueberry crop each year may be freeze-dried, a process by which the water is sublimated directly from the frozen state under conditions of very low absolute pressure. Although freeze dried fruit are considered very good quality from the point of view of nutrition and flavor, the process is used to a limited extent because of its high cost.
Some of the health benefits provided by fruits and vegetables in the human diet have been attributed to antioxidant activity. In antioxidant-rich berries such as blueberries, two major contributors to antioxidant activity are ascorbic acid, also known as vitamin C, and a complex group of phenolic compounds. Much of the antioxidant activity of the phenolics is attributed to a sub-group known as anthocyanins. Anthocyanins are the primary pigments of berries and are responsible for the color of berries. In fact, various anthocyanins are responsible for almost all of the red, purple and blue colors of fruits and flowers. They are known to have strong antioxidant activity (Wang et al., 1996. Total antioxidant capacity of fruits. Journal of Agricultural and Food Chemistry 44: 701-705). Of all berries, blueberries have the highest concentration of anthocyanins, followed by cranberries and strawberries.
Drying of fruits is described in the patent literature and attention is directed to Kraig et al. U.S. Pat. No. 4,515,822 which teaches a method of coating fruit pieces with sugars and gums, then drying rapidly in air above 220.degree. F. to puff and dry the fruit pieces. Koshida et al. U.S. Pat. No. 4,341,803 teaches a method of producing a crisp dry fruit snack by a sequential combination of freeze drying, microwave drying and vacuum drying. Nafisi-Movaghar U.S. Pat. No. 5,000,972 teaches a method of drying fruit without sulfiting. Mazin et al. U.S. Pat. No. 5,188,861 teaches a method of removing natural flavor from dry fruit pieces and introducing a new, substantially different flavor and Durance et al. U.S. Pat. No. 5,962,057 teaches a method of drying mango and pineapple with fresh flavor and crunchy texture. None of these patents deal with the specific problem of drying antioxidant-rich berries while minimizing the loss of antioxidant properties of the dried berries.
Blueberries have recently been the subject of scientific study as potent antioxidants in the human diet (Costantino, L. et al. 1996. Anthocyanin inhibitors of xanthine oxidase. Pharmazie (50):573-574
Antioxidants can be measured by various methods. Known antioxidant compounds may be quantified by standard analytical methods. For example, total phenolics can be measured by the Folin-Ciolacalteu method, using gallic acid as a standard (Velioglu et al. 1998. Journal of Agriculture and Food Chemistry (46): 4113-4117). Anthocyanins may be measured a spectrophotometric methods (Fuleki, T. and Francis, F. J. 1968. J. of Food Science (33):73-83.) Antioxidant activity can also be measured directly be initiating a standard oxidation reaction in vitro, such as the reaction measured by the thiobarbituric acid reaction assay (TBAR) (Buerge and Aust, 1978. Methods in Enzymology (52):302-310), then measuring the inhibition of oxidation conferred by the test sample. Antioxidant activity in a different system can be measured by the DPPH free radical scavenging method (Hu and Kitts. 2000. Antioxidant activity of Echinacea root extracts. Journal of Agricultural and food Chemistry 48: 1466-1472.) DPPH is a stable free radical (2,2-diphenyl-1-picrylhydrazyl) that is scavenged directly by the antioxidant substrate. This method therefore measures inhibition properties at both the initiation and propagation stages of the oxidation reaction.